Method for a lithographic apparatus

ABSTRACT

A method of increasing a depth of focus of a lithographic apparatus is disclosed. The method includes forming diffracted beams of radiation using a patterning device pattern; and transforming a phase-wavefront of a portion of the diffracted beams into a first phase-wavefront having a first focal plane for the lithographic apparatus, and a second phase-wavefront having a second, different focal plane, wherein the transforming comprises: subjecting a phase of a first portion of a first diffracted beam and a phase of a corresponding first portion of a second diffracted beam to a phase change which results in an at least partial formation of the first phase-wavefront, and subjecting a phase of a second portion of the first diffracted beam and a phase of a corresponding second portion of the second diffracted beam to a phase change which results in an at least partial formation of the second phase-wavefront.

This application claims priority and benefit under 35 U.S.C. §119(e) toU.S. Provisional Patent Application No. 61/193,419, entitled “Method ForA Lithographic Apparatus”, filed on Nov. 26, 2008. The content of thatapplication is incorporated herein in its entirety by reference.

FIELD

The present invention relates to a method of increasing the depth offocus of a lithographic apparatus.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a patterning device, which is alternatively referredto as a mask or a reticle, may be used to generate a circuit patterncorresponding to an individual layer of the IC, and this pattern can beimaged onto a target portion (e.g. comprising part of, one or severaldies) on a substrate (e.g. a silicon wafer) that has a layer ofradiation-sensitive material (resist). In general, a single substratewill contain a network of adjacent target portions that are successivelyexposed. Known lithographic apparatus include so-called steppers, inwhich each target portion is irradiated by exposing an entire patternonto the target portion in one go, and so-called scanners, in which eachtarget portion is irradiated by scanning the pattern through the beam ina given direction (the “scanning”-direction) while synchronouslyscanning the substrate parallel or anti-parallel to this direction.

In the semiconductor manufacturing industry, there is an increasingdemand for ever-smaller features and increased density of features.Critical dimensions (CDs) of pattern features are therefore rapidlydecreasing, and are becoming very close to the theoretical resolutionlimit of state-of-the-art lithographic apparatus such as the steppersand scanners as described above. Conventional techniques for enhancingresolution and minimizing patternable critical dimension include:reducing the wavelength of the exposure radiation; increasing thenumerical aperture of the projection system of the lithographicapparatus; and including features smaller than the resolution limit ofthe lithographic apparatus so that they will not be patterned onto thesubstrate, but so that they will produce diffraction effects which canimprove contrast and sharpen fine features of patterns applied to thesubstrate. However, application of such conventional resolutionenhancement techniques may lead to a reduction of depth of focus withinwhich, for example, imaging of desired patterns at or near the limit ofthe resolution capability can be achieved. A reduced depth of focus maylead to pattern defects (for example, the blurring of lines or edges ofpattern features) beyond tolerance when, for example, a residualsubstrate unflatness cannot be compensated for during exposure of thesubstrate.

It is desirable to provide, for example, a method and apparatus thatobviates or mitigates one or more of the problems identified above, orone or more of the problems of the prior art in general, whetheridentified herein or elsewhere. For instance, it is desirable to providea method and apparatus which increases the depth of focus of a patternfeature imaged by a lithographic apparatus.

SUMMARY

According to an aspect, there is provided a method of increasing a depthof focus of a pattern feature imaged by a lithographic apparatus, themethod comprising: illuminating a patterning device pattern, provided bya patterning device, with a radiation beam, the patterning devicepattern comprising a pattern feature that diffracts the radiation beamto form a plurality of diffracted beams of radiation, illuminating aphase modulation element with the diffracted beams of radiationemanating from the patterning device, and using the phase modulationelement to control (e.g., transform) the phase of at least a portion ofradiation constituting each of the diffracted beams of radiation to forma first phase-wavefront having a first focal plane (and a first depth offocus) for the lithographic apparatus, and a second phase-wavefronthaving a second focal plane (and a second depth of focus) for thelithographic apparatus, the first and second focal planes (and depths offocus) being offset relative to one another along an optical axis of thelithographic apparatus, wherein controlling (i.e. transforming) thephase of the radiation constituting at least a portion of the diffractedbeams of radiation comprises: controlling (e.g., transforming) the phaseof a first portion of a first diffracted beam of radiation and acorresponding first portion of a second diffracted beam of radiation sothat the first portions of the first and second diffracted beams ofradiation are subjected to a first phase change which results in an atleast partial formation of the first phase-wavefront; and controlling(e.g., transforming) the phase of a second portion of the firstdiffracted beam of radiation and a corresponding second portion of thesecond diffracted beam of radiation so that the second portions of thefirst and second diffracted beams of radiation are subjected to a secondphase change which results in an at least partial formation of thesecond phase-wavefront.

When the first and second phase-wavefronts combine to form an image on aradiation beam receiving element (e.g. a substrate, a detector, or thelike) the depth of focus of the first and second phase-wavefronts willcorrespond to the combined images through focus to result in anincreased depth of focus in comparison with the situation where no phasemodulation was undertaken.

The radiation beam that illuminates the patterning device may compriseincoherent radiation. The radiation beam that illuminates the patterningdevice may have an illumination mode, the illumination mode having asingle pole centered on an optical axis of the lithographic apparatus.

The patterning device pattern may be a contact hole pattern.

The first portions of the first and second diffracted beams of radiationand the second portions of the first and second diffracted beams ofradiation may be corresponding in that they have the same relativeposition in each respective diffracted beam of radiation.

The first portion of the first or second diffracted beam of radiationand the second portion of the first or second diffracted beam ofradiation may define substantially equal areas when projected onto thephase modulation element.

When projected onto the phase modulation element, the first and secondportions of the first or second diffracted beam of radiation may meet ata center of the first or second diffracted beam of radiation.

Alternate and/or adjacent portions of each diffracted beam of radiationmay be subjected to a phase change which results in the at least partialformation of alternate phase-wavefronts.

A phase of eight, sixteen or thirty two different portions of eachdiffracted beam of radiation may be controlled.

The first phase change may together define (or be defined by) a radialphase distribution. The second phase change may together define (or bedefined by) a radial phase distribution.

The first phase-wavefront and second phase-wavefront may each have adifferent positive degree of curvature; or the first phase-wavefront andsecond phase-wavefront may each have a different negative degree ofcurvature; or the first phase-wavefront may have a positive degree ofcurvature and the second phase-wavefront may have a negative degree ofcurvature.

A phase change for the first portion of the first or second diffractedbeam of radiation may be equal and opposite to a phase change of thesecond portion of the first or second diffracted beam of radiation

There may be little or no overlap between the diffracted beams ofradiation when incident upon the phase modulation element.

The phase modulation element may comprise a controllable region. Thecontrollable region may be controllable to change a refractive index ofthe controllable region. The controllable region may be controllable byselectively heating the controllable region. The controllable region maybe controllable by selectively controlling a shape, position ororientation of the controllable region.

The phase modulation element may be located at or adjacent to a pupilplane of the lithographic apparatus.

The first phase change or the second phase change may be a zero phasechange.

According to an aspect, there is provided a method of increasing a depthof focus of a lithographic apparatus, the method comprising: formingdiffracted beams of radiation by illuminating a patterning devicepattern with a radiation beam, the patterning device pattern comprisinga pattern feature that diffracts the radiation beam, illuminating aphase modulation element with the diffracted beams of radiation, andtransforming a phase-wavefront of a portion of the diffracted beams ofradiation into a first phase-wavefront having a first focal plane (and afirst depth of focus) for the lithographic apparatus, and a secondphase-wavefront having a second focal plane (and a second depth offocus) for the lithographic apparatus, the first and second focal planes(and first and second depths of focus) being offset relative to oneanother along an optical axis of the lithographic apparatus, wherein thetransforming comprises: subjecting a phase of a first portion of a firstdiffracted beam of radiation and a phase of a corresponding firstportion of a second diffracted beam of radiation to a phase change whichresults in an at least partial formation of the first phase-wavefront,and subjecting a phase of a second portion of the first diffracted beamof radiation and a phase of a corresponding second portion of the seconddiffracted beam of radiation to a phase change which results in an atleast partial formation of the second phase-wavefront.

According to an aspect, there is provided a lithographic apparatuscomprising a phase modulation element configured to control the phase ofat least a portion of radiation constituting each of a plurality ofdiffracted beams of radiation from a patterning device to form a firstphase-wavefront having a first focal plane for the lithographicapparatus, and a second phase-wavefront having a second focal plane forthe lithographic apparatus, the first and second focal planes beingoffset relative to one another along an optical axis of the lithographicapparatus, wherein control of the phase of the radiation constituting atleast a portion of the diffracted beams of radiation comprises:controlling the phase of a first portion of a first diffracted beam ofradiation and a corresponding first portion of a second diffracted beamof radiation so that the first portions of the first and seconddiffracted beams of radiation are subjected to a first phase changewhich results in an at least partial formation of the firstphase-wavefront, and controlling the phase of a second portion of thefirst diffracted beam of radiation and a corresponding second portion ofthe second diffracted beam of radiation so that the second portions ofthe first and second diffracted beams of radiation are subjected to asecond phase change which results in an at least partial formation ofthe second phase-wavefront.

The lithographic apparatus may have, where appropriate, one or more ofthe features described above in relation to the methods describedherein.

The controlling of the phase modulation element may be undertaken by acontroller (e.g. a computer or the like).

The arrangement configured to combine the first phase-wavefront and thesecond phase-wavefront may be a lens arrangement.

According to an aspect, there is provided a device manufactured usingthe method or apparatus described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the invention;

FIG. 2 a schematically depicts an illumination mode;

FIG. 2 b schematically depicts a part of a contact hole pattern providedby a patterning device;

FIG. 2 c schematically depicts a depth of focus when the contact hole ofFIG. 2 b is imaged by a lithographic apparatus;

FIG. 3 schematically depicts a distribution of diffracted radiation on aphase modulation element that is located at or adjacent to a pupil planeof a projection system of the lithographic apparatus;

FIG. 4 a schematically depicts a phase distribution provided by thephase modulation element in accordance with an embodiment of the presentinvention, in relation to the diffracted radiation shown in anddescribed with reference to FIG. 3;

FIGS. 4 b and 4 c schematically depict a first phase-wavefront and asecond phase-wavefront, respectively;

FIG. 4 d schematically depicts the phase distribution, in general,provided by the phase modulation element in accordance with anembodiment of the present invention;

FIG. 5 a schematically depicts a part of the illumination mode shown inand described with reference to FIG. 2 a;

FIG. 5 b schematically depicts a part of a contact hole pattern providedby a patterning device;

FIG. 5 c schematically depicts the depth of focus of the contact hole ofFIG. 5 b when imaged by the lithographic apparatus for the part of theillumination mode shown in and described with reference to FIG. 5 a,when subjected to the phase distribution shown in and described withreference to FIG. 4 a;

FIG. 6 a schematically depicts another part of the illumination modeshown in and described with reference to FIG. 2 a;

FIG. 6 b schematically depicts a part of a contact hole pattern providedby a patterning device;

FIG. 6 c schematically depicts the depth of focus of the contact hole ofFIG. 6 b when imaged by the lithographic apparatus for the part of theillumination mode shown in and described with reference to FIG. 6 a,when subjected to the phase distribution shown in and described withreference to FIG. 4 a;

FIG. 7 a schematically depicts a complete illumination mode that is thecombination of the parts of the illumination mode shown in and describedwith reference to FIGS. 5 a and 6 a;

FIG. 7 b schematically depicts a part of a contact hole pattern providedby a patterning device;

FIG. 7 c schematically depicts the combined depth of focus of thecontact hole of FIG. 7 b when imaged by the lithographic apparatus forthe combined parts of the illumination mode shown in FIGS. 5 a and 6 awhen subjected to the phase distribution shown in and described withreference to FIG. 4 a;

FIG. 8 a schematically depicts the depth of focus of a contact hole whenimaged by the lithographic apparatus when the radiation constituting theillumination mode is not subjected to a phase change according to anembodiment of the present invention;

FIG. 8 b schematically depicts the depth of focus of a contact hole whenimaged by the lithographic apparatus when the radiation constituting theillumination mode is subjected to the phase modulation according to anembodiment of the present invention;

FIG. 9 schematically depicts a pattern formed, in an x-y plane (e.g. theplane of a radiation beam receiving element such as a detector or asubstrate) when radiation constituting the illumination mode used toimage the pattern was subjected to the phase modulation shown in anddescribed with reference to FIG. 4 a;

FIG. 10 schematically depicts a more detailed view of an embodiment ofthe phase modulation element shown in and/or described with reference toFIGS. 1 to 9; and

FIG. 11 schematically depicts further detail of the embodiment of thephase modulation element shown in and described with reference to FIG.10.

DETAILED DESCRIPTION

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCDs), thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “wafer” or “die” herein may beconsidered as synonymous with the more general terms “substrate” or“target portion”, respectively. The substrate referred to herein may beprocessed, before or after exposure, in for example a track (a tool thattypically applies a layer of resist to a substrate and develops theexposed resist) or a metrology or inspection tool. Where applicable, thedisclosure herein may be applied to such and other substrate processingtools. Further, the substrate may be processed more than once, forexample in order to create a multi-layer IC, so that the term substrateused herein may also refer to a substrate that already contains multipleprocessed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g. having a wavelength in the range of5-20 nm).

The term “patterning device” used herein should be broadly interpretedas referring to a device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate. Generally, the patternimparted to the radiation beam will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit. Any use of the terms “reticle” or “mask”herein may be considered synonymous with the more general term“patterning device”.

A patterning device may be transmissive or reflective. Examples ofpatterning device include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “projection lens” herein may beconsidered as synonymous with the more general term “projection system”.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more patterning device tables). Insuch “multiple stage” machines the additional tables may be used inparallel, or preparatory steps may be carried out on one or more tableswhile one or more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index, e.g.water, so as to fill a space between the final element of the projectionsystem and the substrate. Immersion techniques are well known in the artfor increasing the numerical aperture of projection systems.

FIG. 1 schematically depicts a lithographic apparatus according to aparticular embodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL to condition a beam PB ofradiation (e.g. UV radiation or EUV radiation);

a support structure (e.g. a support structure) MT to support apatterning device (e.g. a mask) MA and connected to first positioningdevice PM to accurately position the patterning device with respect toitem PL;

a substrate table (e.g. a wafer table) WT to hold a substrate (e.g. aresist-coated wafer) W and connected to second positioning device PW toaccurately position the substrate with respect to item PL;

a projection system (e.g. a refractive projection lens) PL configured toimage a pattern imparted to the radiation beam PB by patterning deviceMA onto a target portion C (e.g. comprising one or more dies) of thesubstrate W; and

a phase modulation element PME located in or adjacent to a pupil planePP of the projection system PL, the phase modulation element PME beingarranged to adjust a phase of at least a part of an electric field ofthe radiation beam.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above).

The support structure MT holds the patterning device. It holds thepatterning device in a way depending on the orientation of thepatterning device, the design of the lithographic apparatus, and otherconditions, such as for example whether or not the patterning device isheld in a vacuum environment. The support structure can use mechanicalclamping, vacuum, or other clamping techniques, for exampleelectrostatic clamping under vacuum conditions. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired and which may ensure that the patterning device is at a desiredposition, for example with respect to the projection system.

The illuminator IL receives a beam of radiation from a radiation sourceSO. The source and the lithographic apparatus may be separate entities,for example when the source is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam is passed from the source SO to the illuminator ILwith the aid of a beam delivery system BD comprising for examplesuitable directing mirrors and/or a beam expander. In other cases thesource may be integral part of the apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, may be referred to as aradiation system.

The illuminator IL may comprise adjusting means AM for adjusting theangular intensity distribution of the beam (e.g. for providing a desiredillumination made in the radiation beam). Generally, at least the outerand/or inner radial extent (commonly referred to as σ-outer and σ-inner,respectively) of the intensity distribution in a pupil plane of theilluminator can be adjusted. In addition, the illuminator IL generallycomprises various other components, such as an integrator IN and acondenser CO. The illuminator provides a conditioned beam of radiationPB, having a desired uniformity and intensity distribution in itscross-section.

The illumination system may encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the beam of radiation,and such components may also be referred to herein, collectively orsingularly, as a “lens”.

The radiation beam PB is incident on the patterning device (e.g. mask)MA, which is held on the support structure MT. Having traversed thepatterning device MA, the beam PB passes through the projection systemPL, which focuses the beam onto a target portion C of the substrate W.In passing through the projection system PL, the beam PB also passesthrough the phase modulation element PME. With the aid of the secondpositioning device PW and position sensor IF (e.g. an interferometricdevice), the substrate table WT can be moved accurately, e.g. so as toposition different target portions C in the path of the beam PB.Similarly, the first positioning device PM and another position sensor(which is not explicitly depicted in FIG. 1) can be used to accuratelyposition the patterning device MA with respect to the path of the beamPB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the object tables MT and WT will berealized with the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of thepositioning device PM and PW. However, in the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in the following preferred modes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to thebeam PB is projected onto a target portion C in one go (i.e. a singlestatic exposure). The substrate table WT is then shifted in the X and/orY direction so that a different target portion C can be exposed. In stepmode, the maximum size of the exposure field limits the size of thetarget portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the beam PB isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT is determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the beam PB isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 a schematically depicts an illumination mode for use in, forexample, applying a pattern to a substrate. The illumination mode may becreated and used by, for example, the lithographic apparatus shown inand described with reference to FIG. 1. The illumination mode consistsof a single pole 2 located on the optical axis of the lithographicapparatus. The illumination mode comprises radiation that is incoherent.

The illumination mode is used to illuminate a pattern provided by apatterning device, for example the patterning device shown in anddescribed with reference to FIG. 1. FIG. 2 b shows a part of the patternprovided by the patterning device. The part of the pattern is a 100 nmbinary contact hole 4. It will be appreciated that FIG. 2 b shows onlyone of the binary contact holes which would make up a whole contact holepattern. The contact hole pattern may, as a whole, comprise a pluralityof binary contact holes. The plurality of contact holes may have, forexample, a pitch of 292 nm. The cross-sectional dimension of the(binary) contact hole, as well as the pitch of the contact holes, aregiven by way of example only.

FIG. 2 c schematically depicts a depth of focus 6 of the lithographicapparatus used with the illumination mode shown in FIG. 2 a to exposethe pattern shown in FIG. 2 b. It can be seen that a region ofrelatively uniform intensity extends along the z-direction (i.e. alongthe optical axis) for approximately 200 nm (i.e. between about −100 nmand +100 nm about a mid-point of maximum intensity).

As discussed above, it is desirable to increase the depth of focus of alithographic apparatus (i.e. an image formed by a lithographicapparatus). Such an increase in the depth of focus may be desirable tocounteract a resolution enhancement technique which may lead to areduction in the depth of focus. Alternatively or additionally, anincrease in the depth of focus may be desirable to increase the rangeover which imaging of a desired pattern at or near the limit of theresolution capability can be achieved. Such an increase in the depth offocus may, therefore, be desirable even if the depth of focus has notbeen reduced by a resolution enhancement technique.

According to an embodiment of the present invention, the depth of focusof a lithographic apparatus (i.e. an image formed by a lithographicapparatus) may be increased by modulating (i.e. controlling) the phaseof one or more portions of one or more diffracted beams of radiationthat are diffracted by the pattern provided by the patterning device. Inparticular, the phase of one or more corresponding portions of thediffracted beams of radiation (i.e. having the same relative position inrespective diffracted beams when projected onto the phase modulationelement) may be subjected to a first phase change which results in thosecorresponding parts combining to form a phase-wavefront having a desiredshape. This phase-wavefront will have a first depth of focus (i.e. afirst focal plane). Other corresponding portions of the diffractedradiation beams may be subjected to a second phase change to help ensurethat those parts combine to form a second, different phase-wavefront.The second phase-wavefront has a second, different, depth of focus (i.e.a second focal plane) to the first phase-wavefront. When the first andsecond phase-wavefronts combine to form an image on a radiation beamreceiving element (e.g. a substrate, a detector, or the like) the depthof focus of both phase-wavefronts will correspond to the combined imagesthrough focus to result in an increased depth of focus in comparisonwith the situation where no phase modulation was undertaken.

A method according to an embodiment of the present invention will now bedescribed with reference to FIGS. 3 to 11.

FIG. 3 schematically depicts the distribution of diffracted beams ofradiation 10 that are incident upon the phase modulation element PMEshown in and described with reference to FIG. 1. Specifically, FIG. 3depicts the footprint (i.e. the projection of) the diffracted beams ofradiation 10 on the phase modulation element PME. The diffracted beamsof radiation 10 are generated as the radiation constituting theillumination mode has passed through, along, between or around (or beenreflected off) the pattern provided by the patterning device, and indoing so being diffracted by the pattern provided by the patterningdevice.

The phase modulation element PME is transmissive, although in anotherembodiment, the phase modulation element may be reflective. The phasemodulation element PME comprises a plurality of controllabletransmissive regions (discussed in more detail below in relation toFIGS. 10 and 11). The refractive index of each region can beindividually controlled to effectively control (i.e. modulate) the phaseof radiation passing through that particular region. FIG. 4 a shows howsuch a phase modulation element PME can be used to independently controldifferent parts of each of the diffracted beams of radiation.

FIG. 4 a shows the diffracted beams of radiation 10 together with thephase modulation applied to each portion 12 of each of the diffractedbeams of the radiation 10 by different regions of the phase modulationelement PME. It can be seen from FIG. 4 a that each of the diffractedbeams of radiation 10 is, where incident upon the phase modulationelement PME, divided into eight different portions 12. Each of the eightportions 12 are circular sectors of equal area. The portions 12 arecircular sectors due to the footprint of the diffracted beams beingsubstantially circular in shape. The portions 12 may have shapes otherthan circular sectors. Each diffracted beam may be divided into greateror fewer than eight portions.

Alternating and adjacent portions 12 have their phase controlled (i.e.transformed) to ensure that after passing through the phase modulationelement the portions combine to form one of two differentphase-wavefronts. This may be achieved by mapping the required phaseonto a desired wavefront. For example, of the eight portions 12 of eachdiffracted beam, the first, third, fifth and seventh portions may havetheir phase controlled such that they combine to form at least a part of(i.e. are mapped onto) a first phase-wavefront, for example the firstphase-wavefront 14 shown in two-dimensions in FIG. 4 b. Referring hackto FIG. 4 a, the second, fourth, sixth and eighth portions of the eightportions 12 of each diffracted beam may have their phase controlled suchthat they combine to form at least a part of (i.e. are mapped onto) asecond phase-wavefront, for example the second phase-wavefront 16 shownin two-dimensions in FIG. 4 c. Referring back to FIG. 4 a, correspondingportions of different diffracted beams of radiation (i.e. having thesame relative position in respective diffracted beams when projectedonto the phase modulation element) have their phase controlled such thatthey combine to form at least a part of (i.e. are mapped onto) the samephase-wavefront as each other. Alternating (e.g. first and second)portions have their phase controlled such that they combine to form atleast a part of (i.e. are mapped onto) alternate (e.g. first and second)phase-wavefronts. Alternating portions that are mapped onto alternatephase-wavefronts may be repeated within, and/or around a center of, eachdiffracted beam of radiation.

Referring to FIG. 4 a, it can be seen that the division of thediffracted beams into eight portions of equal area creates an eight-foldsymmetry. Such symmetry means that areas mapped onto a firstphase-wavefront are not adjacent to areas mapped onto a secondphase-wavefront. As will be discussed below, this helps ensure thatthere is no astigmatism in an image formed by the combination andimaging of the diffracted beams of radiation (and thus the two differentphase-wavefronts) on a radiation beam receiving element (e.g. asubstrate or the like). This means that the phase modulation does notresult in a displacement or distortion of the image in the x-y plane(i.e. the plane onto which the image is projected).

FIG. 4 a shows the phase distribution provided by the phase modulationelement overlaid on top of the diffracted beams of radiation. FIG. 4 dis a more generic depiction of the phase distribution of the phasemodulation element PME. The effect of this phase distribution is nodifferent to that shown in and described with reference to FIG. 4 a. Itcan be seen that in practice the phase modulation may extend across anarea of the phase modulation element that does not just include thoseareas onto which diffracted beams of radiation are incident. In order todivide the diffracted beams of radiation into different portions,regions 12 a of the phase modulation element are appropriatelycontrolled to cause a desired phase change in those portions to map theportions onto different wavefronts. One or more adjacent regions may beassociated with the formation of a first wavefront (i.e. to control thephase of a first portion of a diffracted beam). One or more adjacentregions may be associated with the formation of a second wavefront (i.e.to control the phase of a second portion of a diffracted beam).Individually controllable regions of the phase modulation element maynot have a shape which corresponds to the shape of a portion of thefootprint of a diffracted beam when incident on the phase modulationelement. Instead, a plurality of individually controllable regions ofthe phase modulation element may, together, have a shape whichcorresponds to the shape of a portion of the footprint of a diffractedbeam when incident on the phase modulation element.

It can be seen in FIGS. 4 a and 4 d that the regions of the phasemodulation element which are associated with the formation of a givenphase-wavefront are not located adjacent to one another. Specifically,it can be seen that a side of a region associated with the formation ofa first phase-wavefront is not adjacent to a side of another region alsoassociated with the formation of that first phase-wavefront. Corners orvertices of the regions may, however, be adjacent to one another. Sincethe regions of the phase modulation element control the phase ofportions of the diffracted beams of radiation, it will also beappreciated that portions of the diffracted beams of radiation which areincident on the phase modulation element and which form a givenphase-wavefront are not located adjacent to one another. Specifically,it can be seen in FIG. 4 a that a side of a footprint of a portion of adiffracted beam of radiation which is incident on the phase modulationelement and which forms at least a part of a given phase-wavefront isnot adjacent to a side of another portion which forms at least a part ofthat first phase-wavefront. The result of this is that there is littleor no astigmatism in an image formed using the phase-wavefronts. Theseprinciples are generally applicable, and are not limited to, forexample, the shapes of the phase modulation element regions, diffractedbeams, and portions of those diffracted beams shown in FIGS. 4 a and 4d. For example, in principle any distribution of the illuminationradiation into areas of equal size can be used, where the correspondingdiffraction orders of these areas experience two (or more) differentwavefronts. To avoid distortions in a resultant image, such asastigmatism, however, the areas of the diffracted beams may be equal inthe x axis and/or y-axis and desirably also in the diagonal direction(relative to the x and y axes). This may be achieved by dividing theillumination radiation into eight portions, or sixteen portions, orthirty two portions, etc.

Phase-wavefronts having different shapes will, after passing through afocusing optical system, have different focal points and possibly alsodifferent depths of focus which are located at different points along anoptical axis. The first and second phase-wavefronts described above willhave first and second focal planes and substantially equal depths offocus. According to an embodiment of the present invention, thedifferent focal planes of the two different phase-wavefronts are takenadvantage of. Upon focusing of the two different phase-wavefronts,images formed by the two different phase-wavefronts combine to create anincreased overall depth of focus for the lithographic apparatus. FIGS. 5a-c will be used to explain what happens to radiation constituting afirst portion of radiation constituting the illumination mode whensubjected to the phase modulation discussed above. FIGS. 6 a-6 c will beused to explain what happens to radiation constituting a second portionof radiation constituting the illumination mode when subjected to thephase modulation discussed above.

FIG. 5 a shows a first portion 2 a of radiation constituting theillumination mode shown in and described with reference to FIG. 2 a. Itwill be understood that FIG. 5 a does not, in any way, depict adifferent illumination mode to that shown in and described withreference to FIG. 2 a. Instead, FIG. 5 a is simply depicting a firstportion 2 a of radiation forming the illumination mode that is incidentupon the patterning device. The first portion of radiation 2 a shown inFIG. 5 a is divided into four circular sectors that are separated fromone another and equally spaced from one another. The distribution of thecircular sectors corresponds, at least in part, to the distribution ofcircular sectors discussed above in relation to the phase distributionprovided by the phase modulation element.

FIG. 5 b shows a part of the contact hole pattern 4 illuminated by thefirst portion of radiation shown in and described with reference to FIG.5 a.

The first portion of radiation shown in FIG. 5 a will, after beingdiffracted by the pattern of the patterning device, be incident upon thephase modulation element provided with the phase distribution shown inFIGS. 4 a and 4 d. The first portion of radiation shown in FIG. 5 a willtherefore form corresponding portions in the diffracted beams ofradiation that are incident on the phase modulation element, and whichhave their phase controlled such that the diffracted portions are mappedonto a first phase-wavefront.

FIG. 5 c shows that this first phase-wavefront has a first depth offocus 18. The first depth of focus 18 has a range of substantiallyuniform intensity which extends approximately 200 nm in the z-direction(i.e. along the optical axis). Due to the phase change introduced by thephase modulation element, it can be seen that the first depth of focusin FIG. 5 c is displaced in the negative z-direction by approximately150 nm in comparison with the depth of focus shown in and described withreference to FIG. 2 c (for which the radiation beam was not subjected toa phase change).

FIG. 6 a shows a second portion 2 b of radiation constituting theillumination mode shown in and described with reference to FIG. 2 a. Itwill be understood that FIG. 6 a does not, in any way, depict adifferent illumination mode to that shown in and described withreference to FIG. 2 a. Instead, FIG. 6 a is simply depicting a secondportion of radiation 2 b forming the illumination mode that is incidentupon the patterning device. The second portion of radiation 2 b shown inFIG. 6 a is divided into four circular sectors that are separated fromone another and equally spaced from one another. The distribution of thecircular sectors corresponds, at least in part, to the distribution ofcircular sectors discussed above in relation to the phase distributionprovided by the phase modulation element. In this embodiment, thecircular sectors of the second portion of radiation 2 b are at locationsthat are alternating between the circular sectors of the first portionof radiation 2 a.

FIG. 6 b shows a part of the contact hole pattern 4 illuminated by thesecond portion of radiation shown in and described with reference toFIG. 6 a.

The second portion of radiation shown in FIG. 6 a will, after beingdiffracted by the pattern of the patterning device, be incident upon thephase modulation element provided with the phase distribution shown inFIGS. 4 a and 4 d. The second portion of radiation shown in FIG. 6 awill therefore form corresponding portions in the diffracted beams ofradiation that are incident on the phase modulation element, and whichhave their phase controlled such that the diffracted portions are mappedonto a second phase-wavefront.

FIG. 6 c shows that this second phase-wavefront has a second depth offocus 20. The second depth of focus 20 has a range of substantiallyuniform intensity which extends approximately 200 inn in the z-direction(i.e. along the optical axis). Due to the phase change introduced by thephase modulation element, it can be seen that the second depth of focusin FIG. 6 c is displaced in the positive z-direction by approximately150 nm in comparison with the depth of focus shown in and described withreference to FIG. 2 c (for which the radiation beam was not subjected toa phase change).

In practice, it will be appreciated that the first and second portionsof radiation forming the illumination mode will illuminate thepatterning device and phase modulation element in combination. The firstand second portions of radiation forming the illumination mode willinitially constitute a single beam of radiation, and after beingdiffracted will constitute different portions of the diffracted beams ofradiation. This means that the different depths of focus (associatedwith different phase-wavefronts) shown in and described with referenceto FIGS. 5 c and 6 c will not be independent of one another, but willinstead combine to result in an increased depth of focus. This is shownschematically in FIGS. 7 a-7 c.

FIG. 7 a shows an illumination mode 2 used to illuminate a patternprovided by a patterning device. The illumination mode 2 comprisesradiation having the combined distribution of the first and secondportions of radiation shown in FIGS. 5 a and 6 a. The illumination mode2 consists of a single pole 2 centered on the optical axis of thelithographic apparatus. The illumination mode of FIG. 7 a is thus thesame as the illumination mode shown in and described with reference toFIG. 4 a.

FIG. 7 b shows a part of a contact hole 4 pattern which is illuminatedby the illumination mode shown in and described with reference to FIG. 7a. The part of a contact hole 4 pattern shown in FIG. 7 b is the same asthe part of the contact hole pattern shown in and described withreference to FIG. 4 b. Radiation constituting the illumination modeshown in FIG. 7 a will diffract off, around, along and/or between (orreflect from) the pattern 2 provided by the patterning device and willbe diffracted as shown in and described with reference to FIG. 3. Thephases of different portions of the diffracted beams of radiation willthen be controlled using the phase modulation element as shown in anddescribed with reference to FIGS. 4 a-4 d. The control of the phase mayinvolve controlling the refractive index of adjacent and alternateregions of the phase modulation element upon which portions of thediffracted beams of radiation are incident. The control of the phase issuch that different portions of these diffracted beams are subjected toa phase change which results in the formation of two differentphase-wavefronts. The two different phase-wavefronts have two differentdepths of focus as discussed above in relation to FIGS. 5 c and 6 c.

FIG. 7 c schematically depicts a combined depth of focus 22. Thecombined depth of focus 22 is a combination of the two different imagesthrough focus which result from the focusing of the two differentphase-wavefronts discussed above (i.e. the combination of the depths offocus shown in and described with reference to FIGS. 5 c and 6 c). Itcan be seen that the combined depth of focus 22 has a region ofsubstantially constant intensity extending over a range of about 400 nmin the z-direction (i.e. along the optical axis). Thus, it can be seenthat the depth of focus is significantly increased when using the methodaccording to an embodiment of the present invention. This is seen moreclearly in FIGS. 8 a and 8 b.

FIG. 8 a shows the depth of focus 6 of a contact hole imaged by alithographic apparatus for which no phase change has been undertaken fordiffracted radiation beams. FIG. 8 b shows the depth of focus 22 of acontact hole imaged by a lithographic apparatus for which a phase changehas been undertaken in accordance with an embodiment of the presentinvention as described above. FIGS. 8 a and 8 b in combination show thatby using a method according to an embodiment of the present invention,the depth of focus has been approximately doubled (i.e. from about 200nm to about 400 nm).

FIG. 9 schematically depicts an image 24 formed according to anembodiment of the present invention on a radiation beam receivingelement (for example, a substrate, a detector, or the like). It can beseen that the image is centered in the x-y plane (i.e. the plane of theradiation beam receiving element on which the radiation beam isreceived). The phase modulation referred to above has not resulted inany significant shifting or distortion of (i.e. astigmatism in) theimage in the x-y plane. No astigmatism is present because of theeight-fold symmetry discussed in more detail above.

In order to determine the phase change that should be introduced in thevarious different portions of the various different diffracted beams ofradiation, desired first and second different phase-wavefronts aredefined, or alternatively, phase distributions that result in suchdesired phase-wavefronts are defined. For instance, the first and secondphase-wavefronts (or in other words focus-fronts) can be expressed in ordefined by spherical Zernikes, as is known in the art. Other radialphase distributions may be used. One phase-wavefront may be positive innature, and the other wavefront negative in nature. This means that onewavefront will have a positive degree of curvature, and the otherwavefront a negative degree of curvature. Of course, otherphase-wavefronts are possible. For example, one or both phase-wavefrontsmay have a different positive or different negative curvature, thusresulting in depths of focus located at different points along theoptical axis. One of the two phase-wavefronts may have no curvature, andradiation that will form that phase-wavefront may not be subjected to aphase change. It will be appreciated that more than two differentphase-wavefronts can be formed, for example, three, four, five or morephase-wavefronts, each phase-wavefront having a different focal plane.

An increased depth of focus may be achieved by ensuring that thedifferent phase-wavefronts are curved, one phase-wavefront having anegative degree of curvature and one phase-wavefront having a positivedegree of curvature. Adjacent and/or alternate portions of eachdiffracted beam may be subjected to equal but opposite phase changes(e.g. +x and −x). This results in a maximum possible combined depth offocus when the two phase-wavefronts are combined. This embodiment mayalso be desirable if the phase modulation element can induce a positiveor negative phase change in radiation passing through the phasemodulation element. This is because the maximum possible phase changewill be from a negative maximum to a positive maximum, and this will belarger (approximately double) the phase change range from zero to apositive or a negative maximum.

In the above embodiments, the portions of the footprints of thediffracted radiation beams have been described as being circularsectors. FIG. 4 c shows that the regions may in practice be, forexample, triangles. Other shapes are also possible, for example squaresor rectangles. The embodiments are also not limited to the division ofeach diffracted beam into eight portions. For instance, each diffractedbeam may be divided into greater or fewer than eight portions.Desirably, the number, and/or shape, and/or size, and/or arrangement ofthe portions are such that an image formed using the diffracted beams ofradiation exhibits little or no astigmatism. Desirably, the diffractionof the radiation constituting the illumination mode is such that thereis little or no overlap between the diffracted beams of radiation whenincident upon the phase modulation element. This allows the phase ofdifferent portions of different diffracted beams of radiation to beindependently controlled.

An embodiment of the present invention is particularly applicable toincreasing the depth of focus of at least a part of a pattern feature(e.g. a contact hole) imaged by a lithographic apparatus having apatterning device that provides a contact hole pattern. This is becausesuch patterns have a relatively uniform diffraction pattern in a pupilplane of a projection system of a lithographic apparatus. Such uniformdistribution allows the phase of different portions of the diffractedbeams of radiation to be accurately and independently controlled. Thepattern provided by the patterning device may be or comprise a contacthole pattern. The contact hole pattern may be a staggered hole contactpattern. Other patterns may be provided, such as for example a rotated‘brickwall’ pattern. In general, the pattern provided may be or compriserepetitive structures. The structures may have a limited depth of focus.

The illumination mode used may comprise of a single pole located on theoptical axis. However, other point-symmetric illumination modes may beused, for instance those that are symmetric in either the x-axis, they-axis, or along a diagonal (relative to the x and y axes) with respectto the Figures described herein. Such illumination modes include, forexample, annular, quasar, quadrupole and asymmetric quasar or quadrupoleillumination modes.

The radiation source may provide coherent and/or incoherent radiation.For coherent imaging an increased depth of focus for a contact hole canbe obtained by, for example, transforming the phase of parts of theradiation beam into a phase-wavefront that consists of a positive andnegative curved part (e.g. in a similar manner to that described above).

As will be appreciated for the embodiments described above, in order torealize the advantages of an embodiment of the present invention a phasemodulation element is used. The phase modulation element may beconfigured so that certain parts of the phase modulation element arearranged to change the phase of certain parts of one or more componentsof a radiation beam incident upon (and/or passing through) the phasemodulation element. The configuration may be actively controlled or bepassively provided (e.g. pre-set) in the phase modulation element. Thephase modulation may be undertaken by appropriate control of theconfiguration of, for example, a transmissive phase modulation elementor of a reflective phase modulation element (e.g. a flexible mirroredsurface or a mirrored surface comprising an array of moveable mirroredfacets or the like).

FIGS. 10 and 11 depict specific embodiments of a suitable phasemodulation element. The phase modulation element PME may comprise anoptical element 3100 formed from material substantially transmissive forradiation constituting the radiation beam used in the lithographicapparatus. The phase modulation element may also comprise, or be inconnection with, a controller 3400. An optical path length for a wavetraversing the optical element 3100 is adjustable in response to asignal provided by the controller 3400. The optical element 3100 may bedisposed, for example, in a Fourier transform plane (e.g. a pupil plane)of, for example, the projection system of the lithographic apparatus.Such a location would mean that, in use, the optical element 3100 istraversed by radiation emanating from the patterning device. Anadjustment (i.e. control or modulation) of a phase of a wave traversingthe optical element 3100 may be achieved by applying heat to a region3200 of the optical element 3100, thereby introducing a local change inthe index of refraction of material constituting the optical elementrelative to the refractive index of material adjacent to and surroundingthe region 3200. The application of heat may be achieved by, forexample, transmitting an electrical current through a wire 3300 havingOhmic resistance and being arranged in contact with the region 3200 ofthe optical element 3300. The controller 3400 is arranged to provide the(correct level of) current to the wire 3300 to achieve a desired changein the refractive index of the region 3200 and therefore modulation ofthe phase of the wave passing through the region 3200.

A plurality of, for example, adjacent portions of the optical element3100 may be provided with a corresponding plurality of wires for heatingone, more, or all regions 3200 of the optical element 3100 independentlyfrom any other region 3200. FIG. 11 schematically depicts an example ofsuch an arrangement. FIG. 11 shows the optical element 3100. Adjacentregions 3200-1 up to 3200-44 are disposed in adjacent rows and, in theFigure, from left to right and from top to bottom. Each region 3200 ofthe regions 3200-1 up to 3200-44 is provided with a correspondingheating wire 3300-1 up to 3300-44. FIG. 11 schematically depicts only afew of these heating wires 3300-1 up to 3300-44 for clarity, although itwill be understood that heating wires would in practice be provided foreach or a plurality of the regions 3200-1 up to 3200-44.

The controller 3400 is constructed and arranged so that each or aplurality of wires 3300-1 to 3300-44 can be current-activatedindependently. This enables application of a spatial phase distributionto one or more optical waves (e.g. diffracted beams derived fromradiation forming an illumination mode traversing the optical element3100. As discussed above, in accordance with an embodiment of thepresent invention this spatial phase distribution may be used tomanipulate specific portions of one or more diffracted beams radiationpassing through the phase modulation element in order to, for example,increase the depth of focus of the lithographic apparatus.

It will be appreciated that the phase modulation element may be formedfrom or comprise any suitable number or regions, and that the number isnot necessarily limited to 44. The number of regions may, in general,depend on a desired special resolution of phase change that is requiredin the lithographic apparatus. For example, a ratio of the area of eachof the regions of the phase modulation element to the size of a cleararea in the pupil plane may be between 100 and 1000. The regions havebeen shown in FIG. 10 and FIG. 11 as being substantially square orrectangular in shape. However, the regions may have other shapes, andmay be, for example, triangular, pentagonal, hexagonal, circular orelliptical in shape. Other embodiments of a phase modulation element canbe seen in, for example, US patent application publication no.2008-0123066. Instead of using a plurality of transmissive regions tocontrol the phase of one or more portions of one or more radiationbeams, a plurality of moveable mirrors could be used.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention, the invention being limited by the claims that follow.

1. A method of increasing a depth of focus of at least a part of apattern feature imaged by a lithographic apparatus, the methodcomprising: illuminating a patterning device pattern, provided by apatterning device, with a radiation beam, the patterning device patterncomprising a pattern feature that diffracts the radiation beam to form aplurality of diffracted beams of radiation; illuminating a phasemodulation element with the diffracted beams of radiation emanating fromthe patterning device, and using the phase modulation element to controlthe phase of at least a portion of radiation constituting each of thediffracted beams of radiation to form a first phase-wavefront having afirst focal plane for the lithographic apparatus, and form a secondphase-wavefront having a second focal plane for the lithographicapparatus, the first and second focal planes being offset relative toone another gong an optical axis of the lithographic apparatus, whereincontrolling the phase of the radiation constituting at least a portionof the diffracted beams of radiation comprises: controlling the phase ofa first portion of a first diffracted beam of radiation and acorresponding first portion of a second diffracted beam of radiation sothat the first portions of the first and second diffracted beams ofradiation are subjected to a first phase change which results in an atleast partial formation of the first phase-wavefront, and controllingthe phase of a second portion of the first diffracted beam of radiationand a corresponding second portion of the second diffracted beam ofradiation so that the second portions of the first and second diffractedbeams of radiation are subjected to a second phase change which resultsin an at least partial formation of the second phase-wavefront.
 2. Themethod of claim 1, wherein the first portions of the first and seconddiffracted beams of radiation and the second portions of the first andsecond diffracted beams of radiation are corresponding in that they havethe same relative position in each respective diffracted beam ofradiation.
 3. The method of claim 1, wherein the first portion of thefirst or second diffracted beam of radiation and the second portion ofthe first or second diffracted beam of radiation define substantiallyequal areas when projected onto the phase modulation element.
 4. Themethod of claim 1, wherein, when projected onto the phase modulationelement, the first and second portions of the first or second diffractedbeam of radiation meet at a center of the respective first or seconddiffracted beam of radiation.
 5. The method of claim 1, whereinalternate and/or adjacent portions of each diffracted beam of radiationare subjected to a phase change which results in the at least partialformation of alternate phase-wavefronts.
 6. The method of claim 1,wherein a phase of eight, sixteen or thirty two different portions ofeach diffracted beam of radiation is controlled.
 7. The method of claim1, wherein the first phase change together defines a radial phasedistribution, or the second phase change together defines a radial phasedistribution.
 8. The method of claim 1, wherein: the firstphase-wavefront and second phase-wavefront each have a differentpositive degree of curvature; or the first phase-wavefront and secondphase-wavefront each have a different negative degree of curvature; orthe first phase-wavefront has a positive degree of curvature and thesecond phase-wavefront has a negative degree of curvature.
 9. The methodof claim 1, wherein a phase change of the first portion of the first orsecond diffracted beam of radiation is substantially equal and oppositeto a phase change of the second portion of the first or seconddiffracted beam of radiation.
 10. The method of claim 1, wherein thereis little or no overlap between the first and second diffracted beams ofradiation when incident upon the phase modulation element, or whereinthere is little or no overlap between the plurality of diffracted beamsof radiation when incident upon the phase modulation element.
 11. Themethod of claim 1, wherein the phase modulation element comprises acontrollable region.
 12. The method of claim 11, wherein thecontrollable region is controllable to change a refractive index of thecontrollable region.
 13. The method of claim 11, wherein thecontrollable region is controllable by selectively heating thecontrollable region.
 14. The method of claim 11, wherein thecontrollable region is controllable by selectively controlling a shape,position or orientation of the controllable region.
 15. The method ofclaim 1, wherein the phase modulation element is located at or adjacentto a pupil plane of the lithographic apparatus.
 16. The method of claim1, wherein the first phase change is a zero phase change or the secondphase change is a zero phase change.
 17. A method of increasing a depthof focus of at least a part of a pattern feature imaged by alithographic apparatus, the method comprising: forming diffracted beamsof radiation by illuminating a patterning device pattern with aradiation beam, the patterning device pattern comprising a patternfeature that diffracts the radiation beam; and illuminating a phasemodulation element with the diffracted beams of radiation, andtransforming a phase-wavefront of a portion of the diffracted beams ofradiation into a first phase-wavefront having a first focal plane (and afirst depth of focus) for the lithographic apparatus, and a secondphase-wavefront having a second focal plane (and a second depth offocus) for the lithographic apparatus, the first and second focal planes(and first and second depths of focus) being offset relative to oneanother along an optical axis of the lithographic apparatus, wherein thetransforming comprises: subjecting a phase of a first portion of a firstdiffracted beam of radiation and a phase of a corresponding firstportion of a second diffracted beam of radiation to a phase change whichresults in an at least partial formation of the first phase-wavefront,and subjecting a phase of a second portion of the first diffracted beamof radiation and a phase of a corresponding second portion of the seconddiffracted beam of radiation to a phase change which results in an atleast partial formation of the second phase-wavefront.